Optical information recording medium

ABSTRACT

An optical information recording medium having a structure in which irregular reflection of information light and recording/reproducing light from the reflective layer of an optical information record medium is prevented, thereby reducing the noise appearing on the reproduced image. The optical information recording medium for recording information by holography comprises a transparent substrate, a recording layer where information is recorded by an interference pattern, and a filter layer formed between the transparent substrate and the recording layer and adapted to transmit a light of a first wavelength and reflect a light of a second wavelength. The filter layer of the thus structured optical information recording medium transmits a light of the first wavelength (for example, red light) and reflects a light of the second wavelength (for example, green light). As a result, two lights of different wavelengths can be separated and used for different purposes without being influenced by each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical information recording mediumwherein information is recorded utilizing holography. In particular, thepresent invention relates to a structure of an optical informationrecording medium wherein the generation of diffusive light from areflective surface which causes various adverse effects is preventedwhen the reflective surface provided on the substrate of an opticalinformation recording medium is irradiated with information light andrecording/reproduction reference light.

2. Description of the Related Art

Holographic recording for recording information to a recording mediumusing holography is generally performed by superimposing light holdingimage information and reference light within the recording medium andwriting the interference pattern formed at this time to the recordingmedium. When reproducing the recorded information, the image informationis reproduced by diffraction due to the interference pattern byirradiating this recording medium with reference light.

One structure of an optical information recording medium implemented inthis holographic recording is proposed in Patent Publication “JapanesePatent Laid-Open Publication No. 11-311936”, released on Nov. 9, 1999.The optical information recording medium proposed in this patentpublication comprises a servo pit 3 provided on plastic or glasssubstrate 1, over which aluminum film or the like is deposited to formreflective layer 2, and further comprises hologram recording layer 4composed of recording material and substrate 5 on top of this reflectivelayer, as shown in FIG. 1.

However, in FIG. 1, not only servo light (red) but also informationlight and reference light for recording used when recording andreference light for reproducing used when reproducing (all of which aregreen) reach reflective layer 2 by being irradiated onto the disk andthereby, is reflected and output from incident and output surface A asreturning light. A portion of the light reflected from reflective layer2 is reflected diffusely because reflective layer 2 is not completelyflat. This diffused reflection light appears in reproducing light asscattering noise. This is problematic in that the reproduction imagecannot be correctly detected by CMOS sensor or CCD and because it isextremely difficult to separate this noise. Because servo light is redlight, it can be separated from the green light of information light andrecording/reproduction reference lights, and therefore, scattering noisefrom reflective layer 2 becomes a problem when green light is emitted.

In addition, even when recording, there is a possibility that, wheninformation light and recording/reproduction reference light reachreflective layer 2, are reflected and generate diffused reflectionlight, this diffused reflection light and the emittedrecording/reproduction reference light both generate a differentinterference pattern. This interference pattern is unnecessary and isproblematic in that not only may this become noise during reproductionbut the recording medium will not be able to reach its originalrecording capacity. If the recording medium does not react to red, thiswill not affect the recording capacity of the recording medium will notbe affected even if servo light is reflected slightly diffusely.

SUMMARY OF THE INVENTION

The present invention was made in consideration of these issues, and theobject thereof is to provide an optical information recording mediumstructure which can prevent diffused reflection from the reflectivelayer of the optical information recording medium due to informationlight and recording/reproduction reference light and reduce the amountof noise appearing in reproduction images.

The optical information recording medium according to the presentinvention for recording information using holography comprises atransparent substrate, a recording layer to which information isrecorded by interference patterns, and a filter layer provided betweenthe transparent substrate and the recording layer which passes light ofa first wavelength and reflects the light of a second wavelength. Thisfilter layer passes light of a first wavelength (for example,red-colored light) and reflects light of a second wavelength (forexample, green-colored light), thereby separating lights of two types ofwavelengths.

In addition, the transparent substrate has a servo pit pattern, andfurthermore, a reflective layer is formed on this pattern. Through thisconstruction, light of the second wavelength cannot reach the reflectivelayer.

Furthermore, a polarization direction-changing layer for changing thepolarization direction of light, for example, a layer composed of aquarter-wavelength board, is provided between the recording layer andthe filter layer. Thus, the changing of the polarization direction oflight and the generation of ghost images due to reflective holograms canbe prevented.

A layer composed of dichroic mirror or a layer composed of cholestericliquid crystal can be uses as filter layer. In particular, combinationwith the afore-mentioned quarter-wavelength board layer is effectivewhen using a layer composed of cholesteric liquid crystal as the filterlayer, the reason being cholesteric liquid crystal characteristicallyreflects light of circularly-polarized light of a predetermineddirection and transmits other lights.

Still further, the reflective surface formed on the substrate isbasically a metallic reflection coating but can also be a medium surfacewhich can record or erase as well as reflect light.

A gap layer for smoothing the substrate surface is provided between thefilter layer and the reflective surface. This gap layer, aside fromsmoothing the substrate surface, works to adjust the size of thehologram recorded in the recording layer.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing the structure of conventional opticalinformation recording medium;

FIG. 2 is a diagram showing a first embodiment of the present invention;

FIG. 3 is a diagram showing a second embodiment of the presentinvention;

FIG. 4 is a diagram showing a third embodiment of the present invention;

FIG. 5 is a diagram showing a fourth embodiment of the presentinvention; and

FIG. 6 is a diagram showing the optical system of the vicinity of anoptical recording system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 2 is a diagram showing the structure of an optical informationrecording medium according to a first embodiment. In optical informationrecording medium 101 according to the first embodiment, servo pit isformed on a polycarbonate or glass substrate 1 and is coated withaluminum, gold, platinum, etc. to provide a reflective layer 2.Although, unlike FIG. 1, servo pit is formed on the entire surface ofthe substrate in FIG. 2, it can also be formed cyclically as in FIG. 1.In addition, the height of this servo pit is 1750 Å maximum and issufficiently small compared to the thickness of the substrate and otherlayers.

In addition, a red-colored light transmitting filter layer 6 is providedon substrate 1 with reflective layer 2 and optical information recordingmedium 100 is constructed by sandwiching hologram recording materialwhich is hologram recording layer 4 between this layer 6 and uppersubstrate 5 (polycarbonate, glass, etc).

In FIG. 1, the red-colored light transmitting filter layer 6 transmitsonly red-colored light and does not pass light of other colors.Therefore, because information light and recording/reproducing referencelight are green- or blue-colored light, they are not transmitted throughfilter 6, but become returning light without reaching reflective layer 2and are output from incident and output surface A. This red-coloredlight transmitting filter layer 6 is, for example, a cholesteric liquidcrystal layer, and Chisso Corporation-product CM-33 and the like can beused. When implementing cholesteric liquid crystal layer, it can beplaced within optical information recording medium 101, in an opticalinformation recording medium structure wherein a quarter-wavelengthboard is placed between filter layer 6 (cholesteric liquid crystallayer) and incident and output surface A. It does not have to be placedwithin optical information recording medium 101 if a quarter-wavelengthboard is placed between dichroic mirror 13, hereafter described, andoptical information recording medium 101 as an optical structure. Thisquarter-wavelength board is shifted by a quarter-wavelength forgreen-colored light only and when the green light incident, it becomescircularly-polarized light. However, if light of other colors (forexample, red) is incident, it becomes elliptically-polarized light.

Optical information recording medium 101 according to this embodimentcan be disk-shaped or card-shaped. If it is card-shaped, servo pit isnot necessary. In addition, in this optical information recording medium101, substrate 1 has a thickness of 0.6 mm, red-colored light passingfilter 6 has a thickness of 2 to 3 μm, hologram recording layer 4 has athickness of 0.6 mm, and substrate 5 has a thickness of 0.6 mm. Thetotal thickness is approximately 1.8 mm.

Next, optical operations in the vicinity of optical informationrecording medium 101 are explained with reference to FIG. 6. First,light (red light) emitted from a servo laser is reflected 100% bydichroic mirror 13 and passes through object lens 12. Servo light isemitted to optical information recording medium 101 by object lens 12such as to focus on reflective layer 2. In other words, dichroic mirror13 transmits light of green- and blue-colored wavelengths and reflectslight of red-colored wavelength almost 100%. Servo light incident onlight incident and output surface A of optical information recordingmedium 101 is transmitted through substrate 5, hologram recording layer4 and red-colored transmitting filter layer 6, reflected by reflectivelayer 2, transmitted through filter layer 6, hologram recording layer 4and substrate 5 once again, and output from incident and output surfaceA. The output returning light is transmitted through objective lens 12and reflected 100% by dichroic mirror 13, and servo information isdetected by a servo information detector, which is not illustrated. Thedetected servo information is used for focus servo, tracking servo,slide servo and the like. Hologram material comprising hologramrecording layer 4 does not react to red-colored light, and therefore thehologram recording layer 4 is not affected even when servo light istransmitted through hologram recording layer 4 or diffusedly reflectedin reflective layer 2. In addition, returning light of servo light fromreflective layer 2 is reflected almost 100% by dichroic mirror 13, andtherefore, servo light is not detected by CMOS sensor or CCD14 fordetecting reproduction image and does not interfere with reproductionlight.

In addition, information light and recording reference light generatedby recording/reproduction laser pass through dichroic mirror 13 and areirradiated onto optical information recording medium 101 by object lens11 such that information light and recording reference light generateinterference patterns within hologram recording layer 4. Informationlight and recording reference light are incident from incident andoutput surface A, mutually interfere in hologram recording layer 4, andgenerate interference patterns therein. Subsequently, information lightand recording reference light are transmitted through hologram recordinglayer 4 and incident on red-colored light transmitting filter layer 6.However, they are reflected before reaching the bottom surface of thislayer and become returning light. In other words, information light andrecording reference light only reach reflective layer 2 becausered-colored light transmitting filter layer 6 has characteristicswherein only red-colored light is transmitted.

Second Embodiment

FIG. 3 is a diagram showing the structure of an optical informationrecording medium according to a second embodiment. In opticalinformation recording medium 102 according to the second embodiment,servo pit is formed on a polycarbonate or glass substrate 1 and iscoated with aluminum, gold, platinum, etc. to provide a reflective layer2. With regards to the height of this servo pit being a maximum of 1750Å, it is the same as in the first embodiment.

The structural differences between the second embodiment and the firstembodiment is that, in optical information recording medium 102according to the second embodiment, there is a gap layer 8, a dichroicmirror layer 9 is provided in place of filter layer 6 of opticalinformation recording medium 101, and furthermore, a quarter-wavelengthboard layer 7 is provided between dichroic mirror layer 9 and hologramrecording layer 4.

Gap layer 8 is formed on reflective layer 2 of substrate 1 by applyingspin coat or the like on material such as UV range. Gap layer 8 protectsreflective layer 2 as well as effectively adjusting the size of thehologram generated within hologram recording layer 4. In other words,the interference range of recording reference light and informationlight must be formed to a certain size in the hologram recording layer4. Therefore, it is effective to provide a gap between hologramrecording layer 4 and the servo pit.

Dichroic mirror layer 9 is formed by dielectric multilayer coating(sputtering) wavelength separation filter on gap layer 8. The dichroicmirror according to the second embodiment has characteristics whereingreen-colored light is reflected and lights of other colors (forexample, red-colored) are transmitted.

Quarter-wavelength board layer 7 is formed by spin-coating azobenzene,for example, on dichroic mirror layer 9. The film generated byazobenzene is anisotropic and has characteristics wherein molecules arealigned perpendicular to the emitted polarized light. Aside from this,quarter-wavelength board layer 7 can be formed by implementing rubbingprocessing, as well. When linear polarized light such as P polarizedlight and S polarized light is incident, quarter-wavelength board layer7 has characteristics wherein transmitted light is turned intocircularly-polarized light from linear polarized light, if the angle ofthis linear polarized light is 45° to the axis of the optical system ofthe crystallization in the quarter-wavelength board. Alternately, ifcircularly-polarized light is incident, it is turned into linearpolarized light. In the present embodiment, quarter-wavelength boardlayer 7 works to eliminate ghosts due to reflective holograms(horizontal fringe), and this layer is not necessary if the influence ofghosts can be ignored.

In addition, in optical information recording medium 102, substrate 1has a thickness of 0.6 mm, gap layer 8 has a thickness of 2 to 3 μm,dichroic mirror layer 9 has a thickness of 1 μm or smaller,quarter-wavelength board layer 7 has a thickness of 20 μm or smaller,hologram recording layer 4 has a thickness of 0.6 mm, and substrate 5has a thickness of 0.6 mm. The total thickness is approximately 1.8 mm.

When performing recording or reproduction of information, red-coloredservo light, green-colored information light and recording/reproductionreference light are irradiated onto optical information recording medium102 having a structure such as this. Servo light enters from incidentand output surface A, passes through hologram recording layer 4,quarter-wavelength board layer 7, dichroic mirror layer 9, and gap layer8, is reflected from reflective layer 2 and becomes returning light.This returning light passes through gap layer 8, dichroic mirror layer9, quarter-wavelength board layer 7, hologram recording layer 4 andsubstrate 5 once again in sequential order, and exits incident andoutput surface A. The output returning light is used for focus servo,tracking servo and the like. The hologram material comprising hologramrecording layer 4 does not react to red-colored light, and therefore,hologram recording layer 4 is not affected even when servo light passesthrough hologram recording layer 4 or is diffusedly reflected inreflective layer 2. Green-colored information light and the like entersfrom incident and output surface A, passes through hologram recordinglayer 4, quarter-wavelength board layer 7, is reflected from dichroicmirror layer 9 and becomes returning light. This returning light passesthrough quarter-wavelength board layer 7, hologram recording layer 4 andsubstrate 5 once again in sequential order, and exits incident andoutput surface A. Additionally, when reproducing, reproduction referencelight, as well as reproduction light generated by irradiating hologramrecording layer 4 with reproducing reference light is output fromincident and output surface A without reaching reflective layer 2. Withregards to reproduction light, whether or not it is reflected bydichroic mirror layer 9 is determined by the recording method, namely,recording reflective hologram or transparent hologram.

Optical operations in the vicinity of optical information recordingmedium 102 (object lens 12, dichroic mirror 13 and CMOS, a detector, andCCD14) are the same as that in the first embodiment, and therefore,explanations are omitted.

Third Embodiment

FIG. 4 is a diagram showing the structure of an optical informationrecording medium according to a third embodiment. In optical informationrecording medium 103 according to the third embodiment, a cholestericliquid crystal layer 10 is provided in place of dichroic mirror layer 9in the second embodiment. Other structures are the same as the secondembodiment.

The thickness of this cholesteric liquid crystal layer 10 is also 1 to 2μm, and as with the other embodiments, the thickness of the entireoptical information recording medium is approximately 1.8 mm.

Cholesteric liquid crystal layer 10 is formed by applying, for example,cholesteric liquid crystal CM-33 (Chisso Corporation product), which isa chiral dopant, after forming gap layer (smooth layer) 8, andspin-coating. Cholesteric liquid crystal has characteristics whereinlight is reflected when circularly-polarized light of a predetermineddirection is incident, and light is transmitted when other light, forexample, linear polarized light and elliptically-polarized light, isincident.

In the third embodiment, quarter-wavelength board layer 7 hascharacteristics wherein it is shifted by a quarter-wavelength for onlygreen-colored light. In other words, in quarter-wavelength board layer7, if green linear polarized light (information light and referencelight) is incident, it is changed to green circularly-polarized light,and if red linear polarized light (servo light) is incident, it ischanged to red elliptically-polarized light. Therefore, green-coloredinformation light and reference light which have been changed fromlinear polarized light to circularly-polarized light byquarter-wavelength board layer 7 are reflected in the cholesteric liquidcrystal layer 10 and do not reach reflective layer 2. In addition,red-colored servo light which has been changed from linear polarizedlight to elliptically-polarized light by quarter-wavelength board layer7 pass through cholesteric liquid crystal layer 10, reach reflectivelayer 2, and the returning light is used for focus servo, tracking servoand the like.

Fourth Embodiment

FIG. 5 is a diagram showing the structure of an optical informationrecording medium according a fourth embodiment. In optical informationrecording medium 104 according to the fourth embodiment, reflectivelayer 2 according to the third embodiment is not a mere metallicreflective film as in A1, but is a recording medium which can be addedor overwritten, such as that treated with phase-change film. Otherstructures are the same as the third embodiment, and therefore,explanations are omitted. Therefore, the effects of cholesteric liquidcrystal layer 10 are the same as that in the third embodiment, and thus,explanations are omitted.

In optical information recording medium 100 having the structure in FIG.1, because high light intensity information light and reference light(green-colored light) also converge on this reflective surface whenrecording holograms, ensuring reliability is problematic even whengiving reflective layer 2 write function using the same red-coloredlight as servo light. In addition, the reflection rate of these filmsenabling write or over-write is not very high, and therefore, problemsexist in that the reproduction efficiency of the holograms is reduced.

However, because reflection rate for green-colored light can beindependently set by cholesteric liquid crystal layer 10 which is awavelength separation layer through the method according to thisembodiment, this is advantageous in that reflective layer 11 forred-colored light for acquiring servo can have low reflection rates.

Effects of Each Embodiment

According to each embodiment, because two different wavelengths areseparated efficiently, the two different wavelengths (red-colored lightand green-colored light) can be used for separate purposes without beinginfluenced by each other.

In addition, a polarization direction-changing layer for changing thepolarization direction of light, for example, a layer composed of aquarter-wavelength board, is provided between the recording layer andthe filter layer. Thus, the changing of the polarization direction oflight and the generation of ghost images due to reflective holograms canbe prevented.

Furthermore, according to each embodiment, information light andreference light used when recording and reproducing and reproductionlight do not reach reflective layer 2 or 11, the generation of diffusedlight on the reflective surface can be prevented. Therefore, noisegenerated by this diffused light are not superimposed on thereproduction image nor detected by CMOS sensor or CCD14, and noise canbe detected to a degree enabling at least error correction of thereproduction image. Noise elements due to diffused light become moreproblematic, the larger the multiplicity of the hologram. In otherwords, the larger the multiplicity becomes, for example one thousand ormore, the smaller the diffraction efficiency from one hologram becomes,making the detection of reproduction image extremely difficult if thereare diffusion noise. According to the present invention, thesedifficulties can be eliminated, and therefore, this is effective. Inaddition, by having a structure as such, servo pit can be obtained fromany form of pre-pit structure and is not limited to sample servo, unlikethe conventional optical information recording medium in FIG. 1.Furthermore, the space between pits can be placed without beinginfluenced by hologram size.

Still further, in the second to fourth embodiments, becausepolarizations of incident light and output light of quarter-wavelengthboard layer 7 are orthogonalized and almost all reproduction light canbe detected due to un-illustrated polarization beam splitter implementedin recording/reproduction optical systems, light utilization efficiencyis high and these inventions are optically superior. In addition, thiscombination is extremely effective for eliminating unnecessary straylights such as surface reflections from optical elements generated onthe laser light source side which is not illustrated, compared toquarter-wavelength board layer 7.

Still further, because the allocation of servo pit and hologramrecording are optically separated, recording density never declinesregardless of the pit format implemented. Therefore, sufficientfrequency band can be given to servo control signal and servo accuracycan be made equal to or higher than conventional optical disks.

Still further, according to the present invention, the reflection rateof the reflection film for servo can be selected freely, and thematerial of the reflective film can also be chosen freely. Thus, as inthe fourth embodiment, recording medium which can be written orover-written, for example DVD (digital video disk), can be used asreflective layer 11, and directory information, such as up to which areahologram is recorded and when over-write was performed, can be writtenand/or overwritten without affecting the hologram.

Filter layer, which is the wavelength separation layer, can be formedcomparably thinner, at micron-level, and therefore, the influence ofoptical aberration of object lens caused by differences in thereflective surface of filter layer 6 and 9 and the reflective surface ofreflective layer 2 and 11 can be ignored.

Although the structure and operation of the present invention have beenexplained based on the principle and embodiments, the present inventionis not limited to the embodiments above, and various modifications arepossible within a range which does not deviate from the main object ofthe invention. In particular, in the present invention, althoughred-colored light is implemented as servo light and green-colored lightas recording/reproduction light, it is not limited thereto, andaccording to the characteristics of the medium, combinations of otherwavelengths are also possible. For example, when using chalcogenidematerial as recording material, blue-colored light is implemented asservo light and red-colored light is implemented asrecording/reproduction light. This is because this material reacts tored-colored light.

1. An optical information recording medium for recording informationusing holography, comprising: a transparent substrate; a recording layerwherein information is recorded by interference pattern; and a filterlayer provided between the transparent substrate and the recording layerwherein light of a first wavelength is transmitted and light of a secondwavelength is reflected.
 2. The optical information recording mediumaccording to claim 1 wherein said transparent substrate has a servo pitpattern.
 3. The optical information recording medium according to claim2 wherein a reflective surface is formed on said servo pit pattern ofsaid transparent substrate.
 4. The optical information recording mediumaccording to claim 1 wherein a polarization direction-changing layer forchanging the polarization direction of light is formed between saidrecording layer and said filter layer.
 5. The optical informationrecording medium according to claim 4 wherein said polarizationdirection-changing layer is a layer composed of quarter-wavelengthboard.
 6. The optical information recording medium according to claim 1wherein said filter layer is a layer composed of dichroic mirror.
 7. Theoptical information recording medium according to claim 4 or 5 whereinsaid filter layer is a layer composed of cholesteric liquid crystal. 8.The optical information recording medium according to claim 7 whereinsaid polarization direction-changing layer changes said light of asecond wavelength to a circularly-polarized light of a predetermineddirection and changes said light of a first wavelength to polarizedlight other than the circularly-polarized light of a predetermineddirection.
 9. The optical information recording medium according toclaim 3 wherein said reflective surface is a metallic reflective film.10. The optical information recording medium according to claim 3wherein said reflective surface is a medium surface which can be writtenor over-written in addition to reflecting light.
 11. The opticalinformation recording medium according to claim 3 wherein a gap layer isprovided between said filter layer and said reflective surface tosmoothen said substrate surface.